Conventional gypsum wallboard has been used for over fifty years in the construction of residential and commercial building interior walls and ceilings. Typically, wallboard consists essentially of a gypsum core sandwiched between and bonded to two sheets of facing material (e.g., paper) and is used as a cost-effective replacement of conventional plaster walls. To be commercially profitable, gypsum products, such as wallboard, are typically manufactured by continuous high speed processes. Typically, natural gypsum (calcium sulfate dihydrate) predominately makes up wallboard. Manufacturers mine and transport gypsum to a mill in order to dry it, crush/grind it and calcine it to yield stucco. The reaction for the calcination process is characterized by the following equation:CaSO4.2H2O+heat→CaSO4.½H2O+1½H2OThis equation shows that calcium sulfate dihydrate plus heat yields calcium sulfate hemihydrate (stucco) plus water vapor. This process is conducted in a calciner, of which there are several types known in the art. The stucco can contain one of two forms of calcium sulfate hemihydrate: the α-hemihydrate form and the β-hemihydrate form. These two types of stucco are often produced by different means of calcination. While the β-hemihydrate form is normally used due to its lower cost, either type of calcium sulfate hemihydrate is suitable for use.
Calcined gypsum (stucco) has the valuable property of being chemically reactive with water, and will “set” rather quickly when the two are mixed together. This setting reaction reverses the above-described stucco chemical reaction performed during the calcination step. The reaction proceeds according to the following equation:CaSO4.½H2O+1½H2O→CaSO4.2H2O+heatIn this reaction, the calcium sulfate hemihydrate is rehydrated to its dihydrate state over a fairly short period of time. The actual time required for this setting reaction generally depends upon the type of calciner employed and the type of gypsum rock that is used. The reaction time can be controlled to a certain extent by the use of additives such as accelerators and retarders.
In known manufacturing processes for gypsum wallboard, the setting reaction is facilitated by premixing dry and wet ingredients in a mixing apparatus, such as a pin mixer. The dry ingredients can include, but are not limited to, any combination of calcium sulfate hemihydrate (stucco), fiberglass, and accelerator, and in some cases natural polymer (i.e., starch). The wet ingredients can be made of many components, including but not limited to, a mixture of water, paper pulp, and potash (hereinafter, collectively referred to as a “pulp paper solution”). The pulp paper solution provides a significant portion of the water that forms the gypsum slurry of the core composition of the wallboard. The dry ingredients and the pulp solution contain the basic chemical components of a piece of wallboard.
Conventional methods of preparing gypsum wallboard are well known to those skilled in the art. For example, the dry ingredients and pulp paper solution can be mixed together in a pin mixer. In this manner, the dry ingredients and pulp paper solution create a fluid mixture or “slurry.” The slurry is discharged from the mixer through the mixer's outlet chute or “boot” which spreads the slurry on a moving, continuous bottom facing material. A moving, continuous top facing material is placed on the slurry and the bottom facing material, so that the slurry is positioned in between the top and bottom facing materials to form the board. The board can then pass through a forming station which forms the wallboard to the desired thickness and width. The board then travels along a belt line for several minutes, during which time the rehydration reaction occurs and the board stiffens. The boards are then cut into a desired length and then fed into a large, continuous kiln for drying. During drying, the excess water (free water) is evaporated from the gypsum core while the chemically bound water is retained in the newly formed gypsum crystals.
While conventional gypsum wallboard products have many advantages, it has also long been desired to reduce the cost of manufacturing gypsum wallboard. One method of reducing the cost of manufacturing gypsum wallboard has been to reduce the amount of water used in the manufacturing of the wallboard. Reduction in water reduces the amount of free water left in the wallboard after the setting reaction. A lower amount of free water left in the wallboard results in less drying energy being expended to remove the free water, which in turn saves energy costs associated with drying wallboard (i.e., the fuel cost associated with operating a kiln to dry the wallboard). However, reducing water negatively impacts the manufacturing process by reducing the slurry fluidity, increasing board weight, adversely affecting the paper to core bond, and decreasing the compressive strength of the board.
To ensure that the slurry remains fluid and the weight of the board is not increased, gypsum wallboard is often produced by incorporating aqueous foam into the stucco slurry. The foam comprises foam cells (i.e., bubbles) that create air pockets in the gypsum core of the wallboard, as the slurry sets. Thus, the core density and the overall weight of the wallboard can be controlled by incorporating aqueous foam into the slurry. The foam usually is prepared using foam water, a foaming solution (i.e., soap), and air in any number of mechanical foam generation devices. As the amount of water used in the slurry decreases, the volume of aqueous foam is increased to maintain desired board weights and thickness. While foam can be used for these purposes, the use of aqueous foam has the detrimental effect of reducing the strength of the produced wallboard.
The increased level of foam produces an increased number of foam cells at the paper core interface. Wallboard gets its strength from the formation and the interlocking of crystals of calcium sulfate dihydrate that form during the rehydration process. At the paper core interface, these crystals of calcium sulfate dihydrate interlock with the fibers of the facing materials to form the paper to core bond. While “paper core interface” and “paper to core bond” is used throughout this disclosure, it is appreciated that any facing material can be used to sandwich the gypsum core. Thus, the term “paper core interface” will refer to the interface between the core and any facing material used and the term “paper to core bond” will refer to the bond formed between the core and any facing material used.
The presence of foam cells at the paper core interface causes a decrease in the strength of the paper to core bond, because the foam cells at the paper core interface prevent a uniform paper to core bond from forming. In addition, the stability of the foam solutions used leads to the production of randomly sized air voids which in turn results in the walls between the air voids being sized non-uniformly. The non-uniform size of the air voids and the walls between the air voids leads to a decrease in compressive strength for such gypsum wallboards that are produced using foams. Thus far, efforts to improve the paper to core bond and the compressive strength in such manufacturing processes have only increased the cost, and in some cases the time involved, in manufacturing gypsum wallboard.
For example, the addition of natural polymers, such as acid modified starches, is found to increase the strength of the paper to core bond. Starch gels during the drying of the wallboard and is carried to the paper core interface by the evaporating water. The presence of the gelled starch at the paper core interface causes a stronger bond between the facing material and the core to form. Such natural polymers are expensive and add cost in manufacturing gypsum wallboard.
Another alternative for strengthening the paper to core bond involves the use of additional steps that both increase the cost and time involved in producing wallboard. An example of such an alternative is sandwiching a low density gypsum slurry with foam between two thin layers of higher density gypsum slurry. This involves the added steps of depositing two layers of higher density gypsum slurry (one on the bottom facing material prior to the depositing of the lower density slurry and one on top of the lower density slurry prior to the placement of the top facing material on top of the slurry). The added steps both increase the cost and the time involved in manufacturing gypsum wallboard.
Other methods focus on soap and dispersant chemistries in order to create foaming solutions with a more uniform foam cell structure. The drawback of such chemistries is their effectiveness are largely dependent on the quality of the water that is used, which varies from plant to plant.